METHANE INCLUSIONS IN HYDROTHERMAL QUARTZ FROM THE PRECAMBRIAN-HOSTED, BOX VEIN OF LYONSDALE, NY

The Box Vein of Lyonsdale is noted for its box-like cavities lined with clear quartz. It is hosted by Middle Proterozoic marbles near the western margin of the Adirondack Highlands, and is structurally just below the middle Ordovician Knox unconformity.

Primary saline fluid inclusions from the Box Vein were studied by Garside and Darling (1993, GSA abs.) who determined Th of 151 ± 7°C, and fluids characterized by 17 wt.% NaCl and 9 wt.% CaCl₂. Secondary, single-phase CH₄ inclusions also occur in quartz, and are related to late coatings of pyrobitumen. CH₄ has been confirmed by Raman spectroscopy (at 2910.8 cm^-1). H₂O was detected in one CH₄ inclusion, and is likely a film on the inner walls. Homogenization (L + V = L) of 23 CH₄ inclusions occurred at temperatures of -82.9 to -82.1°C with an average of -82.4°C. Measurements above the CH₄ critical point (-82.6°C) suggest another gas phase (possibly ethane) is present although nothing other than H₂O and CH₄ was detected by Raman spectroscopy.

The inclusion with the lowest Th (-82.9°C) has the highest density, which is 0.1936 g/cm³. Because the inclusions homogenize to the liquid phase, they are denser than the CH₄ critical density of 0.1627 gm/cm³. If CH₄ trapping was coeval with saline brines, and the aqueous inclusions can be modeled as a 26 wt.% NaCl equivalent, then isochores from both (immiscible fluids) constrain trapping pressures and temperatures to 490 ± 80 bars and 172 ± 10°C, respectively. Assuming lithostatic load, this suggests a trapping depth (or stratigraphic thickness) of 1.72 ± 0.28 km and an elevated geothermal gradient of 100°C/km.

CH₄ in Precambrian rocks of New York State has been reported from the Richville Shear Zone (northern NY) by Selleck (2005, NYSGA) who inferred that low-density, organic gases descended into basement rocks by strong reverse pressure gradients caused by dilation during shearing (i.e. seismic-pump model). This model could be applied to the Box Vein as well, but it is also possible that horizontal flow of organic fluids and gases from sedimentary sources (to the south) must be considered as well (e.g. Oliver 1986, Geology). Such a model would require a sloped Knox unconformity to the south (or southwest like that of today) at the time of gas trapping. If so, it is conceivable that the Adirondack dome may have already been formed at the time of CH₄ gas trapping.